Pet Radiotracers

ABSTRACT

The invention relates to novel  18 F-labelled compounds which may be suitable for use as Positron Emission Tomography (PET) radiotracers, and to processes for preparation of such. The  18 F-labelled compounds of the invention comprise a [ 18 F]fluoroalkenyl group.

The invention relates to novel ¹⁸F-labelled compounds and their use asPositron Emission Tomography (PET) radiotracers, and to processes forpreparation of such compounds. The invention further relates to kits forpreparation of the compounds and to compositions comprising the¹⁸F-labelled compounds.

The favoured radioisotope for PET, ¹⁸F, has a relatively short half-lifeof 110 minutes. ¹⁸F-labelled tracers for PET therefore have to besynthesised and purified as rapidly as possibly, and ideally within onehour of clinical use. PET tracers are frequently labelled with[¹⁸F]fluoroalkyl groups to produce [¹⁸F]fluoroalkylated PET tracers.[¹⁸F]fluorohaloalkanes are important reagents for performing O—, N—, andS—[¹⁸F]fluoroalkylations and are commonly used to radiolabel ligands foruse in PET studies. Solid-phase preparations of [¹⁸F]fluorohaloalkanesare described in WO-A-2004/056726.

The PET tracers comprising [¹⁸F]fluoroalkyl groups may however sufferfrom in vivo defluorination wherein the [¹⁸F]fluoride is cleaved offfrom the rest of the tracer. This may lead to decreased signal andincreased background noise signal mainly from uptake of [¹⁸F]fluoride inbone tissue. There is hence a clinical need for development of improvedPET tracers which are more stable in vivo and there is a need forsynthetic methods for their preparation in good radiochemical yield.

In view of these needs the present invention provides new compoundssuitable as PET tracers. The compounds of the present invention providean improved target/background signal ratio and increased specificity andsensitivity.

In a first aspect, the present invention provides a compound comprisinga [¹⁸F]fluoroalkenyl group. The compounds are suitable for use as PETradiotracers. The compound is labelled with ¹⁸F either in the 1- or2-position of an alkenyl group, providing 1-[¹⁸F]-fluoro-1-alkenylatedand 2-[¹⁸F]-fluoro-1-alkenylated compounds.

The compounds of the invention comprise a [¹⁸F]fluoroalkenyl group,rather than a [¹⁸F]fluoroalkyl group, used in the state of the art. Ithas been found that the compounds of the invention to a less degreeundergo in vivo defluorination compared to fluoroalkylated compounds andimproved specificity and an increased target/background signal ratio isachieved. Haloalkenes, i.e. compounds of the invention, cannot undergoan S_(N)2 substitution, and have therefore an increased resistance todehalogenation, i.e. are more stable in vivo than the haloalkanes.

In one embodiment the invention provides ¹⁸F-labelled compounds offormula (I)

wherein

-   X′ and X″ are independently selected from hydrogen and ¹⁸F, with the    proviso that either of X′ or X″ is a ¹⁸F-atom;-   n is an integer from 1 to 5;-   the vector comprises a moiety having affinity for a biological    target; and-   L represents a linker moiety.

A ¹⁸F-fluoroalkyl group of PET tracers of the state of the art can besubstituted with a [¹⁸F]fluoroalkenyl group to provide a ¹⁸F-labelledcompound of the invention.

The vector comprises a moiety that has affinity to a biological target,preferably which accumulates in biological targets due to theirbiological and/or physiological properties and therefore can be used tovisualize biological structures, functions and pathological processes.

The vector may be of synthetic or natural origin, and is preferablysynthetic. The vector has the ability to direct the compound to a regionof a given disease. Preferably, the vector has affinity for the target,such as a biological receptor, and preferably binds to this. On the onehand the vector should have a high affinity for the receptor, and on theother hand it should “stay” on the receptor as long as necessary. Thereceptors may be located in the vascular system, in the extracellularspace, associated with cell membranes or located intracellularly.

The vector can generally be any type of molecule that has affinity for abiological target. All vectors that can be linked to the fluoroalkenylgroup without loosing their affinity to the biological target arerelevant. The vector should be physiologically acceptable and shouldpreferably have an acceptable degree of stability. The vector maycomprise biomolecules, preferably selected from the group consisting ofpeptides, peptoids/peptidomimetics and proteins; oligonucleotides, suchas oligo-DNA or oligo-RNA fragments; oligosaccharides; lipid-relatedcompounds; hormones; synthetic small drug-like molecules; inhibitors;antibodies and antibody fragments; and derivatives and mimetics thereof.

Vectors comprising a peptide may include linear or cyclic peptides, orcombinations thereof. The peptides comprise from 1-150 amino acids andare preferably 3-100mer peptides, and more preferably 3-30mer peptides.By the term “cyclic peptide” is meant a sequence wherein two amino acidsare bonded together by a covalent bond which may be a peptide ordisulphide bond or a synthetic non-peptide bond such as a thioether,phosphodiester, disiloxane or urethane, forming a cyclic bridge (ring).The peptides may comprise 1, 2 or more such cyclic bridges and thenumber of amino acids between two amino acids which are bonded are e.g.3-15.

Suitable peptides for use in the vector include the following, usingstandard symbols for the amino acids:

-   -   somatostatin, octreotide and analogues;    -   peptides which bind to the ST receptor, where ST refers to the        heat-stable toxin produced by E. coli and other micro-organisms;    -   laminin fragments e.g. YIGSR, PDSGR, IKVAV, LRE and        KCQAGTFALRGDPQG;    -   N-formyl peptides for targeting sites of leukocyte accumulation;    -   Platelet factor 4 (PF4) and fragments thereof;    -   RGD-containing peptides;    -   Angiotensin II;    -   Endothelins;    -   Cytokines such as VEGF, EGF, hepatocyte growth factor, nerve        growth factor, interferons, interleukins, platelet-derived        growth factor, tumour necrosis factor, macrophage        colony-stimulating factor and fragments thereof;    -   Chemokines such as MCP-1 and eotaxin;    -   Peptide fragments of α₂-antiplasmin, fibronectin or beta-casein,        fibrinogen or thrombospondin.

Synthetic peptides of the vector may be obtained by conventional solidphase synthesis, as described by Merrifield employing an automatedpeptide synthesizer (J. Am. Chem. Soc., 85: 2149 (1964)).

Suitable oligonucleotides in a vector comprising oligonucleotides arepolymers of ribonucleotides or deoxyribonucleotides comprising between 5and 100 units, preferably between 10 and 30 units. The oligonucleotidesmay contain only the five common nitrogen bases of natural nucleicacids, or they may contain non-natural and/or synthetic bases.

Suitable oligosaccharides in a vector comprising oligosaccharides arepolymers of sugars, containing from three to twenty units, preferablyfrom three to ten units. The constituent sugars are glucose, galactose,mannose, fructose, N-acetylglucosamine, N-acetylgalactosamine or sialicacids, but other sugars, including synthetically modified sugars, may bepresent. The sugar chains may be linear or branched.

Suitable lipid-related compounds in a vector comprising lipid-relatedcompounds are hydrophobic compounds preferably from common buildingblocks of eukaryotic biological membranes, such as phospholipids,glycolipids or cholesterol. Preferably, they are related to or derivedfrom these compounds. Examples of compounds that are derived fromarachidonic acid are prostaglandins and thromboxanes. From phospholipidsare derived lysophosphatidylcholine, diacylglycerol andplatelet-activating factor; from cholesterol, steroids such as cortisol,progesterone, estradiol and testosterone. Retinoids also belong in thisgeneral class of compounds.

Suitable inhibitors enzyme, such as enzyme inhibitors, in a vectorcomprising inhibitors may be naturally occurring proteins such ascystatins, serpins or tissue inhibitors of metallo proteinases (TIMPs,native or modified). They may be of microbial origin, such as leupeptin,semi-synthetic, or synthetic, such as lysine chloromethyl ketone.

Suitable monoclonal antibodies or fragments thereof, in vectorscomprising such antibodies, include: antibodies to the CD-20 antigenexpressed on the surface of B-cells; anti-leucocyte or anti-granulocyteantibodies; anti-myosin antibodies or antibodies to carcinoembryonicantigen (CEA).

Suitable synthetic small drug-like molecules, in vectors comprisingsynthetic small drug-like molecules, include, but are not limited to:estradiol, estrogen, progestin, progesterone and other steroid hormones;ligands for the dopamine D-1 or D-2 receptor, or dopamine transporterssuch as tropanes; and ligands for the serotonin receptor.

In a preferred embodiment, the compounds of the invention comprise alinker, L, connecting the vector to the fluoroalkenyl moiety. In itssimplest form L is a covalent bond or comprises a functional group whichpermits facile conjugation of the vector and the fluoroalkenyl moiety. Lpreferably comprises moieties selected from the group consisting ofamines, amides, thioethers, ethers, sulfones, sulfoxides, sulfides,polyalkyleneglycols, polylactic acids or polyglycolic acid moieties andamino acids, e.g. peptides of 1 to 5 amino acids.

Compounds including a ¹⁸F-fluoroalkyl group, described for use as PETtracers, exist. Examples of such compounds include, but is notrestricted to, benzothiazoles,2-(1,1-dicyanopropen-2-yl)-6-(2-fluoroethyl)methylamino)-naphtalene(FDDNP), the halophenyl nortropane family and the 4-haloethenylphenyltropane family (U.S. Pat. No. 6,843,97) In one embodiment, the inventionprovides similar compounds to these groups of compounds, but comprisinga ¹⁸F-fluoroalkenyl moiety instead of a ¹⁸F-fluoroalkyl group. Preferredsuch compounds are described in the following.

Alzheimer's Disease is a neurodegenerative illness characterised bymemory loss and other cognitive deficits. Amyloid-containing neuriticplaques are a prominent feature of selective areas of the brain inAlzheimer's Disease. In one embodiment the invention provides novelbenzothiazole based compounds which may be used according to the methodsdescribed in WO-A-02/1333 and WO-A-04/083195 for in vivo imaging ofamyloid in brain parenchyma. In this embodiment the invention providesbenzothiazole based compounds of formula (IIa)

wherein R1 to R8 are independently selected from hydrogen, halogen, C₁₋₆alkyl, C₁₋₆ haloalkyl, C₁-C₆-hydroxyalkyl, C₁-C₆-haloalkoxy, hydroxyl,cyano and nitro; R9 and R10 are independently selected from hydrogen,C₁-C₆-alkyl, C₁-C₆-hydroxyalkyl and C₁-C₆-haloalkyl, C₂-C₆-alkenyl andC₂-C₆-alkynyl; and wherein one of the groups R1 to R10 comprises a groupof formula (IIb)

wherein X′, X″ and n are as defined in formula I.

Preferably, R9 or R10 comprises the group of formula (IIb).

A preferred compound according to this embodiment is provided by formula(IIc).

The various benzothiazole compounds of the invention may have utility asPET tracers in imaging of Alzheimers Disease.

In a further embodiment, the invention provides a compound of formula(III)

whereinX′, X″ and n are as defined in formula I. The compounds of formula IIIare similar to2-(1,1-dicyanopropen-2-yl)-6-(2-fluoroethyl)methylamino)naphtalene(FDDNP), but comprise a fluoroalkenyl group instead of a 2-fluoroethylgroup.

In a yet further embodiment, the invention provides compounds which arefluoroalkenyl derivatives of1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxyamide(PK 11195). PK 11195 and its preparation is disclosed e.g. in U.S. Pat.No. 5,998,624.

The compounds of this embodiment have affinity to the peripheralbenzodiazepine receptors (PBR) widely distributed throughout the bodyand which has been associated with numerous biological functions. PBR isinvolved in the regulation of apoptosis, in the regulation of cellproliferation, stimulation of steroidogenesis, immunomodulation,porphyrin transport, heme biosynthesis, anion transport and regulationof mitochondrial functions.

The preferred compounds of this embodiment are identified by formula(IV)

wherein

-   X′ and X″ are defined as in formula (I);-   R′ and R″ are individually selected from hydrogen and lower alkyl,    preferably C₁-C₆-alkyl groups, and more preferably methyl;-   Z represents a halogen atom, preferably chloride, fluoride or    bromide, and more preferably chloride.

Z is positioned in the ortho, meta or para position to the N-containingheteroaryl group and more preferably in the ortho position.

The compounds of formula IV may have utility as PET tracers in in vivoimaging of inflammations.

A preferred compound is identified by formula V:

Viewed from a further aspect the invention provides processes forpreparation of a compound according to the invention comprising a[¹⁸F]fluoroalkenyl group. In one embodiment, the compounds of thepresent invention are prepared by a process based on iodonium chemistry.It has been found that a process comprising fluorinating an iodoniumsalts incorporating an alkene group and an aryl group, i.e. fluorinatingan alkenyl(aryl)iodonium salt with ¹⁸F-fluoride, provides fluorinatedalkenes with very high selectivity over the corresponding fluoroaryls.

The process for preparation of the compounds of the invention byiodonium chemistry preferably comprises fluorinating analkenyl(aryl)iodonium salt of formula (VI),

wherein

-   I⁺-Aryl represents a phenyliodonium salt, wherein the aryl group is    optionally substituted with methoxy, ethoxy, propyloxy,    tert-butyloxy, halides or C₁-C₆-alkyl groups; and    the vector, L and n are defined as in formula I.

The process for preparation of the alkenyl(aryl)iodonium salt of formula(VI) preferably comprises the further steps of:

i) reacting a vector moiety, optionally comprising a protecting group,comprising a functional group, with an alkyne comprising a leavinggroup, to prepare a compound comprising a vector moiety linked to analkyne group;ii) hydrometalating the alkyne group of the compound from step (i) toprepare a metalated alkene;iii) reacting the metalated alkene of step (ii) with an iodoniumtransfer reagent to prepare the alkenyl(aryl)iodonium salt.

To prepare a compound of the invention the next step (iv) isfluorinating the alkenyl(aryl)iodonium salt from step (iii) with¹⁸F-fluoride as disclosed.

The process of preparation is further outlined in Scheme 1.

In step i) L represents a linker as defined for formula (I), comprisinga functional group, which is preferably a good nucleophile, such as agroup comprising an N, O or S atom, e.g. a group selected from a primaryor secondary amine, —SH or OH group. LG represents a leaving group e.g.selected from chloride (Cl), bromide (Br), iodide (I), mesylate (OMs),tosylate (OTs), triflate (OTf) or nosylate (ONs). The base is e.g.selected from trialkylamines, pyridine, dimethylpyridine, potassium orsodium tertbutyloxide, sodium hydride, sodium hydroxide, potassiumhydroxide, potassium carbonate or lithium diisopropyl amide (LDA). Thereaction is preferably carried out in the presence of a suitablesolvent, such as acetonitrile or acetone.

In step ii) M comprises a metal group and HM preferably represents aboron, silicon or tin reagent, such as catechol-borane, wherein thehydrometalation of the alkyne group of the reaction product from step(i) will furnish a 1-boryl, 1-stannyl or a 1-silyl alkene.Alternatively, the metal can be a transition metal such as Pd thatinserts directly into the alkyne group. Conversion from Pd etc. can beachieved by addition of a tin compound such as R′″₃SnSnR′″₃ or R′″₃SnCl,wherein R′″ is an alkyl group preferably methyl or butyl. For themetalated alkene generated in this step the metal group M may bepositioned either in the 1- or 2-position of the alkenyl group,depending on the conditions used.

In step iii) Aryl-I (III) represents a suitable iodonium transferreagent, preferably a trivalent phenyliodonium salt, wherein the arylgroup is optionally substituted with methoxy, ethoxy, propyloxy,tertbutyloxy, halides or C₁₋₆ alkyl groups. The iodonium transferreagent is e.g. selected from Ph-I(OCOCH₃)₂, the Stang reagent offormula PhI(CN)OTf or Koser's reagent of formula Ph-I(OH)OTs. Thegenerated alkenyl(aryl)iodonium salt may be precipitated or crystallisedfrom solution, e.g. by addition of a non-polar solvent, such asn-hexane.

In step iv) the alkenyl(aryl)iodonium salt prepared in step (iii) isfluorinated with ¹⁸F⁻. Fluorination with ¹⁸F⁻ may be effected bytreatment with any suitable source of ¹⁸F⁻, such as Na¹⁸F, K¹⁸F, Cs¹⁸F,tetraalkylammonium ¹⁸F fluoride, or tetraalkylphosphonium ¹⁸F fluoride.Depending on whether the metalated alkene generated in step iii)comprises a metal group in the first or second position of the alkenyl,a 1-fluoro-alkenylated or a 2-fluoro-alkenylated compound is generated.To increase the reactivity of the fluoride, a phase transfer catalystsuch as an aminopolyether or crown ether, for example, 4,7,13,16,21,24hexaoxa-1,1 0-diazabicyclo[8,8,8] hexacosane (Kryptofix 2.2.2.) may beadded and the reaction performed in a non protic solvent. Optionally, afree radical trap may be used to improve fluoridation yields, asdescribed in WO-A-2005/061415. The “free radical trap” is an agent thatinteracts with free radicals and inactivates them. A suitable freeradical trap for this purpose may be selected from2,2,6,6-Tetramethylpiperidine-N-Oxide (TEMPO), 1,2-diphenylethylene(DPE), ascorbate, para-amino benzoic acid (PABA), α-tocopherol,hydroquinone, di-t-butyl phenol, β-carotene and gentisic acid. Preferredfree radical traps for use in the process of the invention are TEMPO andDPE, with TEMPO being most preferred.

The treatment with ¹⁸F⁻ is suitably effected in the presence of asuitable organic solvent such as acetonitrile, dimethylformamide,dimethylsulfoxide, tetrahydrofuran, dioxane, 1,2 dimethoxyethane,sulfolane, N-methylpyrrolidine, at a temperature of for example 15° C.to 180° C., preferably at elevated temperature, such as 80° C. to 150°C., for example around 120° C. On completion of the reaction, the¹⁸F-labelled compound of the invention dissolved in the solvent may beseparated from the solvent by evaporation and/or filtration,alternatively the product mixture is diluted with water, the productconcentrated on a C18 cartridge and the product purified by HPLC priorto evaporation.

Any excess ¹⁸F⁻ may be removed from the solution of the compound of theinvention by any suitable means, for example by ion-exchangechromatography or solid phase absorption. Suitable ion-exchange resinsinclude BIO-RAD AG 1-X8 or Waters QMA and suitable solid phaseabsorbents include alumina. The excess ¹⁸F⁻ may be removed using suchsolid phases at room temperature in aprotic solvents. Any organicsolvent may be removed by any standard method such as by evaporation atelevated temperature in vacuo or by passing a stream of inert gas suchas nitrogen or argon over the solution.

In some cases, it may be necessary to protect functional groups in thevector to avoid unwanted reactions during the radiolabelling process.The process hence optionally includes a first step of protectingfunctional groups, such as amine groups or hydroxyl groups, in thevector moiety. Such protection may be achieved using standard methods ofprotecting group chemistry. After the radiolabelling with ¹⁸F iscomplete, any protecting groups may be removed by methods known in theart. Suitable protection and deprotection methodologies may be found,for example, in Protecting Groups in Organic Synthesis, Theodora W.Greene and Peter G. M. Wuts, published by John Wiley & Sons Inc. Forexample, any amine functionality in the vector may be protected byesters, suitably C₁₋₆ allyl or C₁₋₆ haloalkyl esters, preferably acylesters such as tert-butoxycarbonyl (Boc), or ethers, preferablyC₁-C₆-alkyl ethers, or as amides suitably alkylamides such as formylamide. A hydroxyl group in the vector may be protected as an ether, suchas a silyl ether or alkyl ether, or as an ester. These protecting groupsmay be conveniently removed by hydrolysis, for example in the presenceof acid or base. Such deprotection may be effected using a solidsupported acid or base catalyst that render the need for postdeprotection neutralization unnecessary.

The process steps i) to iv) are hence optionally followed by

v) removal of excess ¹⁸F⁻, for example by ion-exchange chromatography;and/orvi) removal of any protecting groups; and/orvii) removal of organic solvents; and/orviii) formulation of the resultant ¹⁸F-labelled compound of theinvention as an aqueous solution.

In another embodiment of this aspect, the invention provides a processfor the preparation of a compound according to the invention, comprisinga [¹⁸F]fluoroalkenyl group, based on a cross metathesis reaction. Thisreaction includes the transalkylidenation of two terminal alkenes underrelease of ethene, catalyzed by ruthenium carbenoids (Grubbs Catalyst).The transalkylidenation allows the exchange of substituents betweendifferent olefins. The compound is prepared in a process reacting¹⁸F-fluoroethene with a compound comprising a terminal alkene group inthe presence of a Grubbs Catalyst. The compound comprising the terminalalkene group is preferably a vector moiety comprising a terminal alkenegroup. The reaction is outlined in scheme 2, wherein the denominationsare as defined in formula (I).

The Grubbs Catalyst has excellent functional group tolerance and readilycouple ¹⁸F-fluoroethene to the modified vector by means of crossmetathesis under release of ethene. In this embodiment of the inventionthis is used to exchange ¹⁸F-fluoride from ¹⁸F-fluoroethene with ahydrogen of a terminal allene linked to a vector. Cross metathesisreactions are described e.g. in Org Lett 1999, 1, 1951. For syntheticpurposes cross metathesis has found limited use because high yieldformation of desired products depends on one reactant being present inlarge excess. For radiochemistry that will always be the case when¹⁸F-fluoride is used. The Grubbs Catalyst used is a ruthenium carbenoid,either of the first or second generation. The second generationcatalysts are more stable and more active than the original ones. Someexamples of relevant catalysts as disclosed by K. Grela, S. Harutyunyan,A. Michrowska, Angew. Chem. Int. Ed., 2002, 114, 4038 are shown below:

The transalkylidenation of the two alkenes is preferably performed inpresence of a anhydrous, aprotic solvent, such as dichloromethane ortoluene.

Methods for the preparation of [¹⁸F]fluoroethene, one of the startingmaterial in the above reaction, have been previously reported bySubramonia et al. (J. Phys. Chem., 1985, 89, 5051; ibid. 1981, 85, 2493)although the presence of by-products may limit the suitability of thesemethods. ¹⁸F-fluoroethene may also be prepared by a process based onconventional displacement of a leaving group with ¹⁸F⁻ followed byelimination. The following alternative synthetic routes may be used toprepare ¹⁸F-fluoroethene:

1) By treating ¹⁸F-fluoroethyl comprising a leaving group such asbromide or iodide with palladium under reductive conditions.¹⁸F-fluoroethyl comprising one leaving group, such as¹⁸F-fluoroethylbromide, may be prepared by treating a 1,2-disubstitutedalkane with ¹⁸F⁻. In the latter case the alkane is substituted with twoleaving groups, of which at least one is either bromide or iodide, andthe other can be bromide, iodide, chloride, tosylate, mesylate, triflateor nosylate. The reaction is preferably conducted in the presence of aaprotic polar solvent such as acetonitrile.2) By treating an ¹⁸F-fluoroethyl comprising a leaving group, e.g.bromide, iodide, chloride, tosylate, mesylate, triflate or nosylate,with a non-nucleophilic base, such as trialkylamine, dimethylpyridine,potassium or sodium tertbutyloxide, sodium hydroxide, potassiumhydroxide, carbonate or acetate, or diisopropylamide.3) By reacting an N,N-dimethyl ethylamine comprising a leaving group,e.g. bromide, iodide, chloride, tosylate, mesylate, triflate or nosylatewith ¹⁸F⁻ and then oxidise the resulting fluoride-N,N-dimethylethylamine to the corresponding amino oxide, e.g. with hydrogenperoxide. The resulting amino oxide readily undergoes elimination toform ¹⁸F-fluoroethene. Yet another alternative, is to introduce fluorideto a reagent where the amino oxide is in place changing the order of thesequence.

The alternative synthetic routes discussed above to prepare¹⁸F-fluoroethene are outlined in scheme 3, wherein X represents aleaving group e.g. selected from Br, I, Cl, OTs, OMs, ONs or OTf.Preferably X is bromide, such that the reactions go via ¹⁸F-fluoroethylbromide in the first two alternative reactions and bromo-N,N,dimethylethylamine is the starting material in the third alternative.

The second starting material in scheme 2, the vector containing aterminal alkene group, may be commercially available or may be preparedusing published organic chemistry methods known to the skilled in theart. The methods used in synthesising this will depend on which linkerand which vector moieties are used.

In a preferred embodiment, the process for the preparation of thecompounds of the invention are carried out in a one-pot tandem reactionwherein ¹⁸F-fluoroethyl comprising a leaving group is added to asolution of palladium(0) and a Grubbs catalyst. This reaction will leadto formation of an alkene and in situ cross metathesis with the Grubbscatalyst will provide a complex wherein the 1-fluoro-ene group isattached to the catalyst. Addition of a vector containing a terminalalkene group will then lead to a second cross metathesis resulting information of the compound of the invention. The one-pot tandem reactionis outlined in Scheme 4. The leaving group X is e.g. selected from Br,I, Cl, OTs, OMs, ONs or OTf. The ¹⁸F-fluoroethyl compound is preferably¹⁸F-fluoroethylbromide. The vector, L and n are defined as in formula(I).

Before use of the compound of the invention, it may be appropriate toformulate it, for example as an aqueous solution by dissolving thecompound in sterile isotonic saline which may contain up to 10% of asuitable organic solvent such as ethanol, or a suitable bufferedsolution such as a phosphate buffer. Other additives may be added suchas ascorbic acid to reduce radiolysis. Hence, in a further aspect thepresent invention also provides a pharmaceutical composition comprisingan effective amount, e.g. an amount effective for enhancing imagecontrast in in vivo PET imaging, of a compound of the invention togetherwith one or more pharmaceutically acceptable adjuvants, excipients ordiluents, preferably as an aqueous solution.

Conveniently, the compound of the invention could be provided as part ofa kit to a radiopharmacy, PET centre, or nuclear medicine department.The kit may contain a cartridge which can be plugged into a suitablyadapted automated synthesiser. The kit includes means for fluorinatingwith ¹⁸F and may also comprise a column to remove unwanted fluoride ion.The kit preferably includes an appropriate vessel comprising a precursorcompound to be ¹⁸F-fluorinated, connected so at to allow the product tobe formulated as required. The reagents, solvents and other consumablesrequired for the synthesis may also be included together with a datamedium, such as a compact disc carrying the software, which allows thesynthesiser to be operated in a way to meet the customer's requirementsfor radioactive concentration, volumes, time of delivery etc.

Conveniently, all components of the kit are disposable to minimise thepossibilities of contamination between runs and may be sterile andquality assured.

In using the kit, a precursor of the compound of the invention,preferably a precursor of a compound of formula (I) would be convertedto the compound of the invention by ¹⁸F-fluorination.

In a preferred aspect the invention hence provides a radiopharmaceuticalkit for the preparation of a compound according to the invention for usein PET chemistry, which comprises

i) a first vessel containing a precursor of the [¹⁸F]fluoroalkenylatedcompound of the invention;ii) a second vessel with means for eluting the first vessel with ¹⁸F;and optionallyiii) an ion-exchange cartridge for removal of excess ⁸F.

The precursor contained in the first vessel is preferably analkenyl(aryl)iodonium salt of formula (VI).

In a further aspect of the invention, there is provided a method forobtaining a PET image using the compound of the invention. In apreferred embodiment the compound of the invention is used as PETtracers in imaging, diagnosing, for surgical guidance or for monitoringthe effect of treatment.

In particular the invention provides a method for obtaining a PET imageof a subject comprising:

1) administering to the subject an image-generating amount of a compoundof the invention, and2) measuring the distribution of the compound within the subject bypositron emission tomography.

Viewed from a further aspect the invention provides a method forobtaining a PET image of at least a part of a human or animal body,previously administered with a compound of the invention.

Viewed from yet a further aspect the invention provides the use of acompound of the invention for the manufacture of a radiotracer for usein a method for obtaining a PET image involving administration of saidradiotracer to a human or animal body and generation of an image of atleast part of said body.

The invention is illustrated by way of the following non-limitingexamples.

EXAMPLES Example 1 Synthesis of O-(3-[¹⁸F]fluoropronenyl)-L-tyrosine

O—[¹⁸F]fluoroalkenyl)-L-tyrosine as a potential oncologic PET tracer isproposed. The analogous O—[¹⁸F]fluoroethyl (FET) andO—[¹⁸F]fluoropropyl-L-tyrosines (FPT) have previously been reported asalternative PET radiotracers to [¹⁸F]fluoro-2-deoxy-D-glucose (FDG)which unlike FDG, differentiate between tumours and cells associatedwith inflammation.

Two alternative syntheses of O-(3-[¹⁸F]fluoropropenyl)-L-tyrosine arepresented:

Method 1—Based on Iodonium Chemistry

Starting from the commercially available materialN-(tert-butoxycarbonyl)-L-tyrosine methyl ester (1), reaction with 1equivalent of 3-bromopropyne in the presence of potassium carbonate andacetone yields the O-(1-prop-3-yne)-L-tyrosine (2).

Alkyne reduction to a functionalised terminal alkenyl group is conductedby hydrometallation of 2 with an appropriate boron reagent. Reduction ofalkyne 2 to the alkenylboronate pinacol ester 3 is conducted accordingto the method of Shiralcawa et al. (Synthesis, 2004, 11, 1814). In thepresence of catecholborane and a catalytic amount of dicyclohexylborane(ca. 5 mol %) in tetrahydrofuran at room temperature, the terminalalkyne 2 is reduced to the alkenylboronic acid catechol ester.Subsequent transesterification with pinacol over ca. 24 hours yields thealkenylboronate pinacol ester 3.

In the presence of 1 equivalent of iodonium transfer reagent Koser'sreagent, [hydroxyl(tosyloxy)iodo]benzene, the iodonium salt 4, theprecursor for radiolabelling, is prepared from its boronic acid ester indichloromethane solution. The product iodonium salt 4 isprecipitated/crystallised from solution by addition of n-hexane.

Radiolabelling of 4 with no-carrier-added (nca) [¹⁸F]fluoride isconducted in a sealed reaction vessel using acetonitrile. Thenucleophilic [¹⁸F]fluoride source is potassium/Kryptofix 222[¹⁸F]fluoride. Optimisation of the labelling reaction is conducted bycarrying out the radiolabelling at a range of temperatures and differentreaction times, e.g. 40 minutes at 90° C.

Finally, deprotection of the amino acid functionality is conducted athigh temperature in the presence of HCl followed by chromatographicpurification of the O-(3-[¹⁸F]fluoropropenyl)-L-tyrosine 5 radiolabelledtracer.

Method 2˜Based on a Cross Metathesis Reaction

The second labelling strategy described requires the synthesis of[¹⁸F]fluoroethene. The [¹⁸F]fluoroethene prepared is used directly in anolefin cross metathesis reaction with Grubbs 1^(st) or 2^(nd) generationcatalysts in the presence of O-(1-prop-3-ene)-L-tyrosine (6) to generatethe radiolabelled product O-(3-[¹⁸F]fluoropropenyl)-L-tyrosine 7.Deprotection of the amino acid functionality results in the preparationof the O-(3-[18F]fluoropropenyl)-L-tyrosine radiotracer, 5.

Synthesis of O-(1-prop-3-ene)-L-tyrosine (6)

Starting from the commercially available materialN-(tert-butoxycarbonyl)-L-tyrosine methyl ester (1), reaction with 1equivalent of 3-bromopropene in the presence of a potassium carbonateand acetone yields the O-(1-prop-3-ene)-L-tyrosine (6).

Synthesis of [¹⁸F]fluoroethene

Starting from commercially available 1,2-dibromoethane, radiolabellingusing nca [¹⁸F]fluoride generates 1-bromo-2-[¹⁸F]fluoroethane.Radiolabelling of 1,2-dibromoethane with nca [¹⁸F]fluoride is conductedin a sealed reaction vessel using acetonitrile as solvent. Thenucleophilic [¹⁸F]fluoride source is potassium/Kryptofix 222[¹⁸F]fluoride. Optimisation of the labelling reaction is conducted bycarrying out the radiolabelling at a range of temperatures and differentreaction times.

Introduction of 1-bromo-2-[¹⁸F]fluoroethane to a solution oftetrakis(triphenylphosphine)palladium(0) in dichloromethane and in thepresence of potassium acetate generates [¹⁸F]fluoroethene. Optimisationof the reaction is carried out by variation of reaction temperatures andtimes.

The reaction of [¹⁸F]fluoroethene, O-(1-prop-3-ene)-L-tyrosine (6) andGrubbs 2^(nd) generation catalyst results in the cross-metathesisreaction to generate the O-(3-[¹⁸F]fluoropropenyl)-L-tyrosine 7. Thecross-metathesis reactions are conducted in dichloromethane attemperatures from ambient to reflux over a variety of different reactiontimes to optimise the reaction conditions.

Deprotection of the amino acid functionality of 7 is conducted at hightemperature in the presence of HCl followed by chromatographicpurification of the O-(3-[¹⁸F]fluoropropenyl)-L-tyrosine 5 radiolabelledtracer.

1. A compound comprising a [¹⁸F]fluoroalkenyl group.
 2. A compound asclaimed in claim 1 of formula (I)

wherein X′ and X″ are independently selected from hydrogen and ¹⁸F, withthe proviso that either X′ or X″ is a ¹⁸F-atom; n is an integer from 1to 5; the vector comprises a moiety having affinity for a biologicaltarget; and L represents a linker moiety.
 3. A compound as claimed inclaim 2 wherein the vector is a biomolecule selected from the groupconsisting of peptides, peptoids/peptidomimetics and proteins;oligonucleotides; oligosaccharides; lipid-related compounds; hormones;synthetic small drug-like molecules; inhibitors; antibodies and antibodyfragments; and derivatives and mimetics thereof.
 4. A compound asclaimed in claim 1 wherein the linker is a covalent bond or comprisesmoieties selected from the group consisting of amines, amides,thioethers, ethers, sulfones, sulfoxides, sulfides, polyalkyleneglycols,polylactic acids and polyglycolic acid moieties and amino acids.
 5. Acompound as claimed in claim 1 of formula (IIa)

wherein R1 to R8 are independently selected from hydrogen, halogen,C₁-C₆-alkyl, C₁-C₆-haloalkyl, C₁-C₆-hydroxyalkyl, C₁-C₆-haloalkoxy,hydroxyl, cyano and nitro; R9 and R10 are independently selected fromhydrogen, C₁-C₆-alkyl, C₁-C₆-hydroxyalkyl and C₁-C₆-haloalkyl,C₂-C₆-alkenyl and C₂-C₆-alkynyl; and wherein one of the groups R1 to R10comprises a group of formula (IIb)

wherein X′, X″ and n are as defined in formula (I).
 6. A compound asclaimed in claim 1 of formula (III)

wherein X′, X″ and n are as defined in formula I.
 7. A compound asclaimed in claim 1 of formula (IV)

wherein X′ and X″ are defined as in formula (I); R′ and R″ areindividually selected from hydrogen and lower alkyl; and Z represents ahalogen atom.
 8. A process for the preparation of a compound accordingto claim 1 by fluorinating an alkenyl(aryl)iodonium salt with¹⁸F-fluoride.
 9. A process as claimed in claim 8 wherein thealkenyl(aryl)iodonium salt is an alkenyl(aryl)iodonium salt of formula(VI),

wherein I⁺-Aryl represents a phenyliodonium salt, wherein the aryl groupis optionally substituted with methoxy, ethoxy, propyloxy,tert-butyloxy, halides or C₁-C₆-alkyl groups; and the vector, L and nare defined as in formula I.
 10. A process as claimed in claim 8 whereinthe alkenyl(aryl)iodonium salt is prepared by i) reacting a vectormoiety, optionally comprising a protecting group, with an alkyne toprepare a compound comprising a vector moiety linked to an alkyne group;ii) hydrometalating the alkyne group of the compound from step (i) toprepare a metalated alkene; iii) reacting the metalated alkene of step(ii) with an iodonium transfer reagent to prepare thealkenyl(aryl)iodonium salt.
 11. A process as claimed in claim 8optionally followed by a) removal of excess ¹⁸F—; and/or b) removal ofany protecting groups; and/or c) removal of organic solvents; and/or d)formulation of the resultant ¹⁸F-labelled compound of the invention asan aqueous solution.
 12. A process for preparation of a compoundaccording to claim 1 by cross metathesis reacting a ¹⁸F-fluoroethenewith a compound comprising a terminal alkene group in the presence of aGrubbs catalyst.
 13. A process as claimed in claim 12 carried out in aone-pot tandem reaction wherein a ¹⁸F-fluoroethyl comprising a leavinggroup is added to a solution of palladium(0) and a Grubbs catalyst toprepare ¹⁸F-fluoroethene, followed by addition of a compound comprisinga terminal alkene group.
 14. A process as claimed in claim 12 whereinthe compound comprising a terminal alkene group is a vector moietycomprising a terminal alkene group.
 15. A compound as claimed in claim 1for use as a PET radiotracer.
 16. A pharmaceutical compositioncomprising a compound as claimed in claim 1 together with one or morepharmaceutically acceptable adjuvants, excipients or diluents.
 17. Amethod for obtaining a PET image using a compound as claimed in claim 1.18. A radiopharmaceutical kit for the preparation of a compoundaccording to claim 1, which comprises i) a first vessel containing aprecursor of a [¹⁸F]fluoroalkenylated compound of claim 1; ii) a secondvessel with means for eluting the first vessel with a source of ¹⁸F.